Automobile Repair/Diagnostics

Diagnostic skills are not taught in automotive schools or books as much as in the past, for several reasons. First, it is often more efficient for a mechanic to remove and replace a part than to try to repair that part. Second, the parts themselves are not made to be repaired as much as in the past. Granted, the time it would take to repair a transmission may be far more than it would be to replace the transmission, so it may be a more efficient system overall. However, the modern way does have its disadvantages--one of them being less ability to diagnose. In one case, a customer was nearly charged $3000 to replace the transmission in a pickup truck when the problem was a bad spark plug (the plug was not firing properly so the engine computer would downshift looking for more power and would then be told to upshift again because the RPMs were too high). Knowing how to really track down the problem can save you a lot of money.

There are several approaches to diagnosing a problem: 1) using a repair manual 2) relying on your own analysis 3) using the experiences of others with the same problem. I suggest grabbing a repair manual until you know more. As you repair with the guidance of a manual, you will develop an 'intuition' that will guide you to problems. This process won't happen on its own, however--you have to be hungry for it.

The first step in any diagnosis is to gather information. Use your senses and ask questions: when does the car make that noise? What does it sound like? How does it feel? Where is the sound coming from? Does it happen at all speeds? Does the car do anything at all?

The second step in diagnosis is to grab a manual armed with that information and look for the troubleshooting charts. I believe that factory manuals have great information, but are not as novice-friendly as the manuals from the auto parts stores. For my own vehicles, I try to get the factory manual (for the most authoritative information) as well as the Haynes (or similar) manual. If you only get one, maybe just the Haynes.

Please note that advanced mechanics will often not need this step, as the problems will be obvious to them. After doing enough repairs, the different systems fit together and do not seem foreign, separate, confusing, or threatening.

Holden / Opel / Vauxhall Astra

Symtoms diagnostics and repairs for common faults of the Astra Mk3 C14NZ engine. Refer to a Haynes Manual for details concerning the actual repair.

Astra Engine temperature erratic usually low

This is usually caused by the thermostat stuck open. Either luckily or by design thermostats usually fail safe - open. On a C14NZ the thermostat is behind the camshaft belt casing and requires the camshaft belt and pulleys to be removed so it cannot be easily tested. New thermostats are remarkably cheap from generic parts supplier, and can be fitted in the upper radiator hose as an alternative to their proper placement. Haynes does not describe this procedure. Remember to fit the thermostat the correct way around,, that is with the longer piece facing the engine.

First check the fuse. Fuse # 11. Second test the Cooling fan thermostatic switch. Removing this will let all the coolant out, observe safety precautions described in the Haynes manual - cool engine and catch coolant. The switch often fails open circuit, use a continuity tester to check if switch closes when heated in a pan of water just before boiling point.

Thirdly check that 12Volts is being supplied to the fan motor, if not, broken wires can be the cause. Lastly prepare to replace the fan assembly, remove it and check it before buying a new one.

Symptoms such as misfires and stalls until engine is running at > 3000 RPM and even then runs rough. Firstly check the spark plugs for excessively large gaps, eroded electrode or dirt. Replace if necessary. Otherwise on the C14NZ engine this is often caused by oil in the distributor cap. The distributor is mounted horizontally on the end of the camshaft and the oil seal allows small amounts of oil into the distributor that then causes short circuits. This can be cured during the early stages by wiping the inside of the distributor cap with a clean rag. Later stages of deterioration burn carbon tracks in the cap and these can be cleaned off, but a new cap is required. If a lot of oil leaks into the distributor the whole unit has to be replaced, it is not possible to buy component parts of the distributor.

The C14NZ engine may run for long periods and then it falters and perhaps stalls. In many cases it will restart and run normally for a while. This fault is infuriating if the period between failures is weeks since you can never be sure that the problem is fixed and it can be caused by numerous faults. First check the failure mode, if the rev counter drops to zero before the engine stops it is most likely to be the Engine Control Unit or the crankshaft position sensor. The C14NZ ECU is pretty robust and the CPS is more lightly to fail. Surprisingly the CPS can be intermittent, but it will eventually fail completely. It is simple to change, but cannot be bought as an individual component. If the rev counter continues to reflect the engine speed the problem is in the High Tension circuit, most likely the plug leads.

The C14NZ engine should run at 600 RPM at idle, if it hunts, increases in speed and then may cut out completely, first check the air filter is in good condition. Otherwise the most likely fault is the hose pipe between the Multec CFI single port injection system and the MAP (Manifold Absolute Pressure) sensor. Check the pipe has not come loose or been damaged.

Engine runs normally under light load but misfires at full throttle. Firstly check the condition of the spark plugs. Then look for signs of a rich mixture, check fuel consumption. An over-rich mixture can be due to a broken Coolant Temperature Sensor or wiring. The sensor operates the dashboard temperature gauge, check readings are sensible. A broken sensor failure is the equivalent of running with the choke on. The ECU thinks the engine is cold. The sensor connector is exposed and rather insecure. The sensor should read about 1600 Ohms at room temperature. See Haynes manual for replacement procedure.

Usually caused by a worn gear linkage. Inside the engine compartment, the top side to side link is made of plastic and seems to be the "weak link". With an assistant in the car, have them try to select 2nd gear, if there is side to side play replace the link. This is not described in the Haynes Manual - details follow.

Jeep Cherokee XJ / Grand Cherokee

DTC 11 on the 4.0L Inline 6 MPI "Power Tech HO" (High Output) engine indicates a fault with the CPS (Crankshaft Position Sensor). The Haynes manual describes the electrical checks to diagnose a faulty unit before purchasing and fitting a replacement.

The CPS on the 4.0 Liter automatic uses one 11mm (*) bolt, the manual uses two. Access is possible from the underside of the engine. The sensor is mounted on the back of the clutch bell housing with little clearance between it and the bulkhead. It can be felt but not seen from underneath the vehicle, and seen but not accessed from the top of the engine. Use a socket with an omni directional extension and a couple of straight extensions so that the ratchet is free of the bodywork. The torque setting is fairly low, and once the socket is in place removal does not require much effort. Replacing the sensor is more difficult, the bolt must be started in its thread by hand and there is barely enough room to use your thumb. Use the socket and extensions to tighten to the correct torque setting.

(* on a Feb 97 UK spec manual the bolt heads mike up at 10.7mm across flats so there's a bit of slack. If not corroded in they come out easily enough using an 11mm socket, as did the one I measured. I don't know whether there is a special tool in exactly the right size to prevent rounding off the hex, but I still have one that's seized in to sort... TS)

The 4.0L Inline 6 MPI "Power Tech HO" engine may develop coolant leaks when the engine core plugs become corroded. Corrosion is often caused by running the engine without anti-freeze or corrosion inhibitors in the cooling system.

Access to the front core plug is possible without removing the manifold. Remove the core plug by locating the weaker corroded part and puncturing it with a center punch or a screwdriver. The core plug is made of fairly soft material, but be very careful not to push the plug into the engine block, if this happens the manifold will have to be removed to recover it. Once the core plug is pierced with the screwdriver lever it out until an edge is exposed and then use a Mole grip to remove it.

Refit using a socket that fits the new core plug loosely, 32 mm or similar. Ensure that the core plug is square on the engine block, add sufficient socket extension bars so that they are free of the manifold and the push it using a large piece of wood as a lever. Once the core plug is engaged feel it to check that it is square, if so drive it in with a shorter extension and a hammer (be sure to use a solid steel hammer, not a sand filled or lead "dead blow" type, as the impact of these hammer types is not sufficient to drive in the frost plug), if not use use a smaller socket to drive one side rather than the other to square it up. Stop when the core plug flange is flush with the engine block. Refill the cooling system with rust inhibitor and anti-freeze.

Frost plug sizes are 2", except certain 1987 Renix models which have both 2" as well as 1" frost plugs.